Well, like I said, some heads were gonna explode...
Looks like the MOPAR Police are on the scene, Officer 72bluNblu reporting for duty!
First off, "Anyone that repairs metal with fiberglass should have their car confiscated" is just a super D-bag statement. Do you have any idea that there are people out there that can't throw $100 bills around to fix their cars? I bet if you took a poll to see how many FABO members have made a fiberglass repair back in the day, you'd get a whole bunch of guys saying "Yep, I sure did".
Second, "It will speed up the decay of the remaining metal" is just a fear statement. Perhaps you missed the part where I laid out some advice for the OP to put money away every month until he has enough for a replacement pan in a year? Are you really proposing that adding fiberglass will cause such catastrophic decay to the surrounding floor pan, in 12 months, that the car could not be saved? Carbon steel corrosion occurs when the relative humidity of the air is 70% to 80%. These conditions are met or exceeded in Virginia (which is where the OP seems to reside) for 3 out of 12 months in a year. So, you believe that in 1 year the amount of decay would somehow be multiplied exponentially with the addition of fiberglass? I would welcome any evidence, supported by chemical molecular data, that this would be the outcome.
Third, "The fact that you go out of your way to say it's a temporary fix and not to take passengers indicates you already know you're giving terrible advice that will compromise the integrity and safety of the car", would suggest that I am purposely trying to put the OP in harm's way by offering my advice. I would never, ever, make any recommendation that would put another person in danger. Life is precious and fleeting. And, are you seriously taking the stance that some fiberglass in a 3'x3' space would have a disastrous effect on the structural integrity of a 3500lb automobile? Not taking passengers was a bit of a joke that maybe someone might put their foot through the floor. My bad for not spelling that out.
Fourth, "If you find yourself seriously considering repairing metal with fiberglass, it means you're in over your head and do not have the skills to make the needed repairs. Walk away and take it to someone that can do it right before you screw it up worse", that can go 2 ways. If directed to the OP, way to shame someone who might not have the finances to take their car to an $80-$100/hour shop. If directed to me, I'm certainly not going to elaborate on my skill set to prove my "worthiness" to the likes of you. My build is proof enough.
Fifth, "In a very short period of time you'll have to do it all again, and if you didn't have the money to fix it right the first time how are you going to have the money to do it twice", where do I even start with this? Who are you? Honestly, who are you to dictate to the OP that if he "doesn't have the money" to do it YOUR way the first time, then don't bother???? If you are such a high roller-big shot-restoration God then put your money where your mouth is. YOU buy the floor pan and have it shipped to the OP's address. Hell, I'll split it with you. OP says the new pan is $500. You do $250, I'll do $250. That way you can sleep easy at night knowing that there is a car driving around out there that hasn't been hacked up by some amateur, and will still "maintain the structural capabilities of the unibody chassis" as you seem to be so worried about.
The OP made no reference to having any welding equipment, or abilities, in his post. He clearly stated that he needed a solution other than "just buy them". So, I offered up a TEMPORARY solution, followed up by a permanent repair when financially allowable. It's up to the OP to take whatever advice he feels is best and discard the rest.
I am in full agreement that the BEST possible solution is to replace the floor pan with new metal.